Sound transmission and reproduction



June 1, 1937. M TE 2,081,904

SOUND TRANSMISSION AND REPRODUCTION Original Filed March 2, 1927 2 Sheets-Sheet l IN V EN TOR.

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w ATTORNEY June 1937- 1.. M. APPLEGATE SOUND TRANSMISSION AND REPRODUCTION Original Filed March 2, 1927 2 Sheets-Sheet 2 is V1704 xuvazvoais AuAA Q/sA/VM/ INVENTOR.

' yy gs w ATTORNEY Patented June 1, 1937 UNITED STATES PATENT OFFICE Lindsay M. Applegate,

Seattle, Wash., assignor,

by mesne assignments, to Collins Radio Company, Cedar Rapids, Iowa,

Delaware Original application March 2, 1927, Serial Patent No. 1,941,591, January 2, 19

a corporation of Divided and this application December 29, 1933, Serial No. 704,539

2 Claims (Cl. 179-1) My invention relates broadly to sound transmission and reception and more particularly to a system for efficiently transmitting and reproducing sounds over the entire audible frequency 5 range.

This application is a division of my copending application Serial No. 172,084, filed March 2, 1927, for Sound transmission circuits now Patent No. 1,941,591, granted Jan. 2, 1934. r In the present development of the art, the

in a restricted volume or as mechanical vibrations in apparatus, are transmitted by electrical undulations or oscillations, and are, in transmission, carried through successive devices to the place of reproduction of the sound transmitted.

Each of such devices is adapted to perform its individual function without regard to the frequency of the oscillations or undulations impressed upon it. Wherefore, if adequate provision be made for the range of all audible frequencies, it will be madepossible to effect reproduction of the sound vibrations of nearly the same form as those of the original sound which is to be transmitted or reproduced. i

So far as regards the sufiiciency of individual apparatus for transmission of the entire range of sound frequencies employed, the situation is the same whether the apparatus consists of the simplest combination of transmitter and receiver or of the most elaborate combination or operative succession of transmitters, lines, broadcasting stations and receiving apparatus. which has been or may be devised. The actual limitations of individual apparatus for adequately transmitting all the frequencies occurring in audible sound are recognized in the art, wherein considerable effort has heretofore been put forth by others to extend the ranges of frequencies of the devices in common use.

Recognizing the actual deficiencies in the instrumentalities used at present in the art, I have invented means for overcoming and substantially removing the aforesaid limitations, and thereby providing a system for the transmission of sound which is applicable to the'wide st range or spectrum of audible frequencies.

Sound waves of any form may be represented electrically or mechanically by different combinations of sinusoidal waves of various frequencies. It is generally conceded that practically all sounds audible to the human ear are represented by combinations of waves of frequencies occurring substantially within the minimum audible sound vibrations in the air at large or 4 transmission of sound within V limit of four cycles per second and the maximum limit of thirty-two thousand cycles per second. In order for a mechanical device, for instance, to be capable of producing or responding to any given frequency, its functioning parts must have 5 certain mechanical characteristics as regards mass and elasticity. In electrical oscillations, on

the other hand, certain limitations in electrical characteristics must be observed if the device is to operate successfully within a given range of frequencies. These limitations are of such definite character that apparatus usually cannot be made to operate successfully, if at all, over the entire range of audible frequencies. As a matter of fact, for illustration, a good audio transformer will seldom satisfactorily transform electrical oscillations of a range more extensive than from about cycles per second to about 3000 cycles per second, or expressed as a ratio, of one to thirty. Consequently, since the entire range of audible frequencies is considered to be a range of 4 to 32,000 cycles per second (or expressed as a ratio, of 1 to 8,000), the limitations of the transformer with a range, for example, of one to thirty is obviously inadequate for transforming currents of all the frequencies which a complete reproduction of sound requires. This limitation of the individual transformer to a frequency range of one to thirty remains about the same regardless of the position of the range in the frequency scale. In other words, a transformer could be one designed with a range of from 4 to cycles per second, or one from 1000 to 30,000, for examples, but the ratio would be substantially constant in every instance.

By way of illustrative example, let it be assumed that we desire to transmit sound within all the frequencies from 4 to 32,000 cycles per second, and that the. satisfactory range possible in a single transformer is of the ratio 1 to 20, that ratio being substantially the correct one that is practically available. For that purpose, selection is made of one transformer having a range of from 4 to 80, of a second transformer having a range of from 80 to 1600, and of a third having a range of from 1600 to 32,000. These three transformers having the respective ranges indicated and operating in conjunction will com stitute a device sufficient for transforming curtransformers is true of other instrumentalities which are or may be employed in sound transmission and reproduction. Such limitations are also encountered in the transmission system as 55 a whole, the transmission line, the receiving system and particularly in the sound reproduction apparatus.

One of the objects of my invention is to provide a sound transmission and reproduction system in which a plurality of modulation circuits at the transmitter are designed for eflicient modulation of energy over different but overlapping bands of audible frequencies.

Another object of my invention is to provide a multiple channel transmission system in which each channel is designed to efliciently transmit differing but overlapping bands of audible frequencies and impress such frequencies upon a multiple unit sound reproducer in which the several units coact to efliciently reproduce all of the audible frequencies over the entire frequency spectrum transmitted by the several channels.

A further object of my invention is to provide a sound transmission system constituted in part by transmission lines and space radio links operative over differing but overlapping modulation frequencies for impressing upon a receiving means the combined effect of the entire frequency spectrum for reproduction by a multiplicityof sound reproducers each operative over differing but overlapping ranges of audible frequencies.

Another object of my invention is to provide a signal transmission and reception system including a multiplicity of transmission channels which are composite in construction, that is including line wire transmission circuits and space radio links therein where each channel is de- Signed to be efiiciently responsive under differing but overlapping ranges of audible frequencies with means connected with all of the channels of a radio receiving station for integrating the effect of the transmitting signaling energy and efliciently reproducing the signaling energy conveyed by the composite channels.

Still another object of my invention is to provide a sound transmission and reproduction system in which the reproduction of sound is provided effected by a multiplicity of sound reproducers each efficiently responsive over differing but overlapping ranges of audible frequencies, the composite signaling "channels including both line wire and space radio links extending from a sound transmitter to the reproducing apparatus where each of the channels are efficiently responsive to differing but overlapping ,ranges of audible frequencies.

A further object of my invention is to provide a broadcasting system in which an oscillation system is separately modulated by a plurality of modulation circuits, each efficiently responsive over different ranges of audible frequencies for effecting the transmission of sound. over sound channels which are operative over different frequency ranges of the transmitted energy wherein the different channels are connected to a multiplicity of reproducers each of which are eificient-' ly responsive over different ranges of the transmitted signaling energy for integrating the effects of all of the transmitted frequencies.

A still further object of my invention residesin a system for the efllcient transmission of sound from a transmitting station to a receiving station wherein provision is made for the division of the entire sound frequency spectrum into separate sections, where each section is particularly emcient to a particular range of audio frequencies, the said device being effective at the microphone circuit through line wire transmission channels, space radio transmission channels, signal receiving circuits and in the actual sound reproducers which integrate the effects of all of the transmitted signaling frequencies.

Other and further objects of my invention will be set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

Fig. 1 shows a circuit arrangement for a transformer system employed in the transmission circuit of my invention; Fig. 2 shows a series of characteristic curves illustratin ,the relative performance of the three transformers illustrated in Fig. 1 when these transformers'care conjunctively operating; Fig. 3 illustratesijanother arrangement of parts in a transmission circuit according to my invention; Fig. 4 shows a modified circuit arrangement for the circuit shown in Fig. 3; Fig. 5 shows a still further modified form of circuit arrangement for the transmission system of my invention; and Fig. 6 diagrammatically shows the entire transmission system of my invention embodying various combinations of the circuits illustrated in the preceding figures for the efficient transmission of signals over composite paths including both line wire and space radio links.

Referring more specifically to the drawings, A, B, and C, indicate, in each figure, individual sound transmitting circuits or instrumentalities, A being a circuit or instrumentality for the transmission of sound whose frequencies are limited to a frequency range of from, for example, 4 to 80 cycles per second of time, B being a circuit or instrumentality limited to a frequency range of from 80 to 1600 cycles per second, and C being a circuit or instrumentality limited to a frequency range of from 1600 to 32,000 cycles per second.

It is for clarity repeated that the number of cir-- cuits may be varied. The frequency range limits of each circuit may also be varied, and also the capacity (range) of the several circuits may overlap one upon another, provided that the circuits collectively cover the entire range-of sound frequencies, of, for example, from 4 to 32,000

specified.

In the art, it is known that certain devices can be used satisfactorily over a much more ex.- tended range of frequencies than others. For example, an electron tube can, in a single unit, amplify satisfactorily a range of frequencies greater than necessary for ordinary sound representations. The same thing is partially true of certain other devices. One of the instrumentalities that appears to be especially limited in the range it can cover satisfactorily is the transformer. In the use of the word transformer here, I consider it as a combination of aprimary coil, a secondary coil and an interlinked magnetic circuit, usually of iron or other magnetic metal.

Fig. 1 illustrates the fundamental provisions of my invention. An input circuit transmitting the entire frequency spectrum is connected to a. transformer system devised to selectively transmit separate frequency bands in the spectrum with minimum loss and distortion. In addition to the natural properties of the individual transformers in responding to frequencies within a limited band, condensers 24, 25, 26 and 21 are provided in parallel with the windings of transformers 2| and 22 in order to improve the selectiveness of the elements of the system. An output circuit is provided in which the component frequency bands are integrated to reproduce the entire frequency spectrum transmitted.

Fig. 2 is a graph of the operative curves of Curve 34 represents the effect of condensers 24 and 25, and curve 36 that of condensers 26 and 21. There is also a curve 38 which represents the resultant performance of all the transformers with their condensers. In this figure, the curves are plotted in coordinates having abscissae expressed in frequency cycles per second and ordinates expressed as transformer secondary voltage.

In the foregoing specification, it is assumed that frequencies of the range of from 4 to 32,000 cycles per second are required for complete reproduction of the entire range of audible sounds, and for the purposes of explanation of my invention, the whole range is divided arbitrarily into three partial bands of frequencies, each partial band being of such extent that the ratio in each band of the lowest frequency to the highest frequency is twenty. It should be understood that this total range, 4 to 32,000, and the divisions thereof into three partial bands is adoptedbyway of example, for purposes of present illustration only. Therefrom, it should be understood that the range of frequencies allotted for use in an apparatus may be in practice either greater or less than that herein assumed. and that the number of divisions into partial bands of frequencies may be accordingly varied from a plurality of two to any greater plurality preferred.

In making the division of the total range into partial bands of frequencies, it is not necessary to preserve perfect uniformity of ratio between the outside frequency limits of the respective individual bands; but they may be of any relative width that is consistent with the requirements of the apparatus used. It is not to be considered that the division ordinarily between the frequency bands is perfectly sharp. Actually the ranges of frequencies over which the several instrumentalities operate will overlap somewhat at the boundaries between successive bands. This is illustrated in Fig. 2 which shows the overlapping of the bands for three transformers illustrated in Fig. 1.

In actual operation, each transformer will afiect the operation of the others,.both by their presence in the circuits as impedances and by the partial transformation of frequencies accommodated by the other bands.

In Fig. 2, curves representing the secondary voltages of the transformers for a given impressed primary voltage at frequencies within the ranges of the transformers, are shown. Thevertical ordinates therein represent transformer second- 4 ary voltage and the abscissae, frequency in cycles per second.

In Fig. 2, it will also be seen that while transformer 2|, whose performance is represented by curve 3|, transforms all the frequencies between 4 and 80 cycles per second, it operates also with somewhat decreased effect at frequencies less than 4 and higher than 80. The curve shows the maximum effect of transformer 2| between 4 and 80, with a decrease practically to zero at about 100. Similarly, transformers 22, curve 32, starts to be effective at somewhat less than 80, increasing to a maximum somewhere near the middle of its. range, and decreasing in effect to practically zero somewhat beyond 1600, the upper boundary of its range. Transformer 23, curve 33, begins below its lower boundary, and continues to be effective somewhat above its highest rated frequency 32,000,

It will be observed that the shapes of the curves are preferably different from each other, depending upon the relative position of the transformer in the combination. Transformer 23, having no by-pass condenser in parallel with it, as 2| and 22 have, should have uniform performance over part of its range as shown in Fig. 2. The several transformers cooperate in the regions where the functioning of the transformers overlaps. In Fig. 2 again, curve 34 shows the assumed effect of condensers 24 and 25 in Fig. 1, and curve 36 shows the effect of the condensers 26 and 21. The efiects of these condensers are to reduce somewhat the output of the transformers they parallel, and so the values given by these curves should be subtracted from the values of the curves 3|, 32, and 33. The resultant curve 38, in this illustration a straight line, may be considered the ideal form, representing as it does, uniform performance of the combination of transformers over the entire range of audible frequencies.

The curves in Fig. 2 are intended for illustration of the principle involved, and are not intended to show a precise representation of either values or forms of performance curves such as are likely to be obtained in practice. In Fig. 2, moreover, some effects have been omitted for convenience. and only the most prominent factors in operation have been included in the curves.

The general form of the curves in Fig. 2 illustrate in a general way, the action of any three instrumentalities functioning cooperatively over any range of frequencies, divided into three A circuit operating with effect over the range of frequencies for which it is indicated, and overlapping into the B band. The B circuit, overlapping into the A and "0 bands, permits a gradual increase of effect in the adjacent circuit with a resultant gradual shifting of the burden from one circuit to another.

In regard to the transmission of sound over electrical circuits, I have already pointed out that some devices can transmit a much wider range of frequencies than others. For some instrumentalities operating with several others in the same complete circuit for transmission, it may be desirable to use three'or more units to cover the entire range, while for some others, only two units will suffice,-and for others, only one need be used for all the frequencies.

In use, the principles described above in general may be subject to great variation, and a great many different combinations of various instrumentalities' are possible, as illustrated in Figs. 3, 4, 5, and 6.

In Fig. 3, three electron tubes are shown delivering their outputs to three transformers 2|, 22, and 23. The primary coils H, l2, and 13 are separate and distinct, and electrically isolated. series as shown previously in Fig. 1. The electron tubes may be considered as detectors or amplifiers operatingwith one or individual radio circuits, and it should be understood that the coils,

as shown, are susceptible of operation for three The secondary coils are connected in one primary electron tube and three secondary electron tubes.

Fig. 4 illustrates a modified form of my invention wherein the three separate coils are shown on one hand as in Fig. 3, but all interlinked with a single magnetic circuit constituting transformer 54. and Y56, with the condenser 58, comprise a single secondary circuit. In effect, this combination results in a change from three partial ranges of frequencies to two partial ranges. The coils 8|, 82, and 83 correspond to the A, B, and C bands respectively, while, on the other hand, coils 55 and 56 are intended to serve two divisions to cover the entire range allotted to three coils before, coil 55' for A and part of "B, coil 56 for the rest of B and all of C". In this illustration, there are shown in order from left to right, units of frequencies for various functions. First, the three electron tubes with the three coils 8|, 82, and 83; next, the single magnetic circuit; then, the two coils 55 and 56; and, finally, the input circuit of the electron tube.

In Fig. 5, another combination for changing from three to two range divisions is illustrated. Said circuit shows electron tube 91 of the A band which delivers its output through coil 9I to the transformer comprising coils 9I, 92, and 95, and the included magnetic circuit. Electron tube 98 of the "3 band delivers its output through coils 92 and 93 to both transformers; and electron tube 99 of the C band delivers its output through coil 94 to the transformer comprising coils 93, 94, and 96 and the included magnetic circuit. Coil 95 receives the transformed energy from coils SI and 92, thereby getting all the A frequencies and part of the "B frequencies. Col] 96 likewise receives the "C" frequencies and part of the "B. The electron tubes I00 and IOI, together operate on the same total range of frequencies as the electron tubes 91, 98, and 99 together do. In the drawings, the electron tubes I00 and IOI are represented as two modulator tubes, operating together to modulate the output of the oscillator tube I02, the power source and usual connections being, of course, omitted. In this arrangement as in all the-others, the transformers can operate equally well receiving energy from two units and delivering to three, as in the way indicated in the figure under consideration.

Fig. 6 illustrates certain of the variations of which my invention is susceptible in application. As schematically shown in Fig. 6, sound energy originating at a transmitting station is transmitted through separate control circuits over composite paths which are individually efficiently responsive to the particular frequency ranges impressed upon the difierent paths. These frequency ranges differ but are overlapping and are conveyed over independent paths which may be composite in character. That is to say, the individual paths may include both line wire and space radio links. The energy is ultimately rendered effective upon an amplification system operative to amplify effectually by virtue of associated circuit elements over a broad frequency range. Independent electromagnetic sound reproducers, each efliciently responsive over different but overlapping frequency'ranges, are connected to the output of the amplification system for insuring the reproduction of the entire frequency spectrum transmitted over the composite paths from the transmitting station. While I have shown certain forms of coupling The coils, on the other hand, numbered 55 elements employed in the transmission circuit of Fig. 6, it is to be understood that any of the coupling means shown in the preceding figures may be substituted for the coupling means illustrated in Fig. 6. In Fig. 6, three transmitters I03, I04, and I05 are used for the entire frequency range. The energy from the transmitters transferred by the transformer I09 is delivered to two power amplifiers H2 and H3, in a manner similar to the transformation illustrated in Fig. 5. The output of the amplifiers is transmitted through transformers H4 and H5, each connected with one of the transmission lines H6 and III. In this regard, it may be pointed out that under some conditions, it may be desirable to adjust two or more transmission lines each to a restricted band of frequencies in the same manner, for example, as previously described for transformers. It should be understood in this connection that a transmission line is simply one of the various instrumentalities used in transmission of sound included in the scope of my invention. The transmission lines II 6, and III deliver energy to the transformers I I8 and H9, which furnish voltage to 'the modulator tubes I20 and I 2I. These modulate individually radio frequency oscillators I 22 and I23, each of which is equipped individually with the radiating or transmitting equipment I24, I26,and I25, I21. The radiating or transmitting equipment is indicated as either including antennae, constituting part of the space radio link, or wire connections, constituting part of the composite circuit. The receiving devices for the two bands are I28, I30, and I29, I3I, with detectors I32 and I33. The two detectors together deliver their respective outputs through coils I34 and I35 to the composite transformer I36, which includes the output coil I31. The power amplifier I38 handles all frequencies, as does the composite transformer I39, which again divides the total range of frequencies between the two coils I40 and HI, which deliver the output of the two transformers to the electromagnetic sound reproducers I42 and I43, each efilciently responsive to differing but overlapping ranges of fre- In the foregoing illustrations, I have shown all units as transformer-coupled, as this is the preferred method. This preference, however, need not preclude the use of the aforesaid so-called resistance and impedance methods or other methods of coupling between power units, where such connections are ordinarily included with devices now in common use.

While I have described my invention in certain of its preferred embodiments, I desire that it be understood that modifications may be made and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent, of the United States is as follows:

1. In a high fidelity sound transmission system, the method of efliciently transmitting all frequencies constituting the sound which comprises transmitting ,differing but oyerlapping ranges of frequency through parallel paths adapted to the individual frequency ranges, integrating said ranges to produce again the complete sound, further transmitting other difiering but overlapping ranges of frequency through paths adapted to said last mentioned frequency ranges, integrating said ranges to again produce the entire frequency spectrum of the sound, and ultimately energizing a plurality of sound reproducing devices of differing but Overlapping frequency characteristics after one of said integrating operations.

.2. In a high fidelity sound transmission system, the method of efficiently transmitting the entire frequency spectrum of a sound, which comprises the transmitting of difiering but overlapping ranges of frequency through parallel paths adapted to the individual frequency ranges, integrating the energy over the several frequency ranges and impressing the entire frequency spectrum upon paths adapted to frequency ranges differing from said first mentioned frequency ranges. 4

LINDSAY M. APPLEGATE. 

